As a turbocharger, well known are a wastegate-type one and a nozzle vane-type one. The wastegate-type turbocharger is mainly for improving engine power; but the nozzle vane-type turbocharger contributes not only toward power improvement but also toward exhaust gas clarification, and recently, in particular, it has become mounted also on diesel engines. The member that constitutes the latter nozzle vane and constitutes an exhaust guide for guiding exhaust gas to a turbine is manufactured mainly by the use of a stainless steel plate, for example, a heat-resistant steel plate of SUS310S or the like. As a special case, Patent Reference 1 describes an invention of manufacturing such an exhaust guide assembly with a high-chromium high-nickel material through precision casting and machining.
FIG. 1 shows an exploded view of one embodiment of members that constitute an exhaust guide of a nozzle vane-type turbocharger. These are a drive ring 1, a drive lever 2, an intermediate nozzle ring 3, a nozzle vane 4 and an outer nozzle ring 5; and the nozzle vane 4 comprises plural vanes 6 to constitute it and vane shafts 7 to support the respective vanes 6. These members 1 to 5 are concentrically assembled and set on the upstream side of the turbine of a turbocharger; and the assembly forms an exhaust guide that guides an exhaust gas to the turbine of a turbocharger through the center opening 8 of the nozzle vane 4. The shafts 7 of the respective vanes 6 of the nozzle vane 4 rotate all in the same direction; and in accordance with the degree of the rotation, the open area (aperture) of the center opening 8 surrounded by the vanes 6 is increased and decreased. When the speed of engine revolution is low, then the displacement is low and the exhaust pressure is low, and in that condition, the open area of the center opening 8 is narrow; but when the speed of engine revolution is increased and the displacement is thereby increased, then the member is driven to broaden the open area. Accordingly, the case having such a nozzle vane is so driven that the speed of the exhaust gas to be led into a turbine is varied in accordance with the speed of engine revolution, or that is, the exhaust gas speed is increased when the speed of engine revolution is low but is lowered when it is high, as compared with a case not having the nozzle vane.
The necessary material characteristics of these members individually differ as follows:
[Drive Ring 1 and Drive Lever 2]
These members are for accurately controlling the aperture of the nozzle vane, working with an actuator; and in general, these are manufactured by blanking with a press, and are required to satisfy fine blanking capability (precision blanking workability) such that the blanked faces could be all shear faces. In their service environment, in addition, the temperature may increase up to about 500° C., and therefore their high-temperature strength in a middle temperature range is important.
[Intermediate Nozzle Ring 3 and Outer Nozzle Ring 5]
These both have location holes for smoothly rotating the vane shafts 7. The outer nozzle ring 5 has a part of ring forging (burring) into a shape that corresponds to the shape of a turbine, in the center opening. Accordingly, these are required to have good machinability and press-formability. These are members serving also for guiding exhaust gas, and are therefore required to keep good high-temperature strength and oxidation resistance even though exposed to high temperatures of about 800° C.
[Nozzle Vane 4]
The nozzle vane 4 is for controlling the open area of an exhaust gas route. Therefore, this is all the time exposed to the exhaust gas running through it, and is exposed to the highest temperature (800 to 900° C.) among the members. Accordingly, this is required to have high-temperature strength enough to resist the pulsating pressure of exhaust gas and to have high-temperature oxidation resistance for smooth driving even at high temperatures. Because of those necessary characteristics, heat-resistant steel plates of SUS310S or the like are generally used for it, but SUS310S steel plates have poor workability.
As in the above, the necessary material characteristics of exhaust guide members of nozzle vane-type turbochargers individually differ for the respective members, and therefore, in general, different steel materials are used for the individual members and different processes are employed individually for them. However, when the members formed of different materials are assembled into a nozzle vane-having exhaust guide assembly, then the difference in the thermal expansion coefficient between the members and the difference in the degree of the formed oxidation scale therebetween may interfere with smooth aperture control of the open area in the exhaust gas route that is the intrinsic function of the nozzle vane-type turbocharger. This problem could be solved when all the exhaust guide members are formed of the same material (steel of the same type); however, a material capable of simultaneously and sufficiently satisfying the above-mentioned, individually different characteristics is unknown. Accordingly, at present, the respective members are formed of different materials that individually satisfy the respective necessary characteristics.
Patent Reference 1 describes an invention for manufacturing an exhaust guide assembly of turbocharger according to a lost wax casting method of using a special high-chromium high-nickel heat-resistant steel that contains Pb, Se and Te. In the invention, the main machining comprises cutting and polishing, and therefore, steel shaping may be omitted and the problem of shapability necessary for steel may be evaded therein. However, the steel contains special additive elements and precision casting is employed for it, and therefore this requires a special manufacture process inevitably with poor producibility and cost increase, as compared with a case of manufacturing exhaust guides in an ordinary production line. In case where a steel plate of SUS310S is used for a member required to have high-temperature oxidation resistance to a further higher level, surface treatment of steel chromizing (treatment for diffusing and penetrating chromium into the surface of steel) or the like may be effective, but this is problematic in that the production process is inevitably complicated and its cost must increase. The chromizing treatment is described in Patent Reference 2.    Patent Reference 1: JP-A 2002-332862    Patent Reference 2: JP-A 6-10114